Abstract Karst terrains and their specific landforms, such as sinkholes and caves, have been thoroughly studied from the second half of the nineteenth century. However, karst genesis remains a puzzling issue to this day. The results of the recent studies of ocean floor and the results obtained by drilling deep oil boreholes have raised doubts about the existing explanations of the karst landforms development and encouraged the emergence of new views on this subject matter. According to the new hypothesis, the majority of karst landforms were formed at great depths beneath sea level where dissolution of carbonates increases dramatically. Underwater dissolution first caused the formation of karst depressions and the primary network of karst conduits elongated along the existing fractures. This process was followed by further expansion of the conduits and the formation of spacious caves due to the water regression and the action of turbulent flows. It is considered that the introduction of the new concept would accelerate the development of karstology and improve the principles and methods for solving numerous practical problems such as the abstraction of quality drinking water and the research of oil, gas and bauxite deposits.
Key words Karst genesis · Landforms · New hypothesis · Dinarides
Introduction
Karst specific features have always attracted the attention of researchers and lead them to think about its genesis. From the late nineteenth century, starting from the pioneering work of Cvijić (1893), widely regarded as the father of modern karstology, to this date a large contribution has been made to understand karst from the geomorphological, hydrological, geological and hydrogeological point of view. The most comprehensive synthesis of previous karst studies is given by Ford and Williams (1989, 2007). Also, a great contribution was given by the following authors: Sweeting (1972); Jennings (1985); Dreybrodt (1988); White (1988); LaMoreaux and LaMoreaux (1998); Palmer et al. (1999); Klimchouk (2000); Gunn (2004); Bakalowicz (2005); Waltham et al. (2005); Andreo et al. (2010). Introduction and further development of karstology have expanded the circle of
scientists and experts who take part in finding answers to a myriad of practical problems, from drinking water supplies, construction of dams and reservoirs, to water protection in general.
Karst covers approximately 12 % of the earth’s land surface and about 25 % of the world population depend on karst aquifers for water supply. The specific nature of karst terrains and complex geological composition, have always posed difficulties to exploring and solving scientific problems in this field. The problems are complex especially when studying highly karstified terrains, and to this date no appropriate methods have been established which would deal with some of the key issues related to water circulation in these terrains. The researchers apply a number of scientific techniques that enable terrain analysis from the surface, from the inside (speleology) and from a distance (remote sensing). However, the complex nature of these problems usually generates insufficiently reliable results and problematic solutions in practice.
The paper presents new views on the karst development which have been encouraged by the discovery of sinkholes and karstified formations far below the present sea level (Land et al. 1995; Michauda et al. 2005; Milovanović 1965). The areas at such great depth could not have been exposed to the effects of atmospheric water which have been so far considered a major agent in the development of most surface and subsurface karst features.
According to the new hypothesis, karst depressions (sinkholes, uvalas and poljes) and some karst conduits are formed beneath sea level, more precisely beneath the lysocline where carbonate solubility dramatically increases.
The hypothesis presented in this paper is mainly exemplified by classic karst of the Dinaric area (41–45°N, 13–19°E). This is due to the marked diversity of karst landforms found in these terrains, but also to facilitate the comparison with the existing fundamental postulate on karst development which is based precisely on the Dinaric karst (Cvijić 1893, 1918). Thickness of the carbonate rocks in the External Dinarides can reach more than 5 km, and the continuous development of karstification to great depths has been registered as well. For analysis of genesis of other karst types (cockpit karst, hypogenic karst, karst in
hot arid and cold regions, karst in evaporites) there was no enough space, but it will be probably done in one of the following papers.
The subject matter covered by this paper is selected as a universal problem. This concept is not in favour of numerous scientific hypotheses so it should not be fitted at all costs in other unproven postulates.
Karst landforms–theoretical background
Karst is a geological term which refers to a set of specific morphological forms of landscape that are the result of interaction between a number of factors, primarily water and water–soluble rocks. Therefore, karst forms are developed only in terrains made of soluble rocks, commonly limestones and dolomites, but also in terrain made of gypsum, anhydrite and halite rocks. Due to the solubility of carbonate rocks (limestones and dolomites), tectonic faults are expanded and secondary porosity of rocks is increased. It causes high permeability of rocks and disappearing of surface water from karst terrains. The absence of a drainage network from surface separates karst terrains from other terrains composed of impermeable rocks. Instead of normal river valleys, specific surface (karren, sinkholes, uvalas, poljes, dry valleys) and subsurface (caves) geomorphologic features are formed conditioning water runoff mostly below the terrain surface.
Karst is a subject of numerous scientific fields, whereas Jovan Cvijić and his followers founded a separate discipline called karstology dealing particularly with karst terrains.
Surface karst forms
The surface of karst terrains is usually characterized by a diversity of karst landforms that provide for a special look of these terrains. Most typical and often described karst forms are: karren, sinkholes, uvalas, poljes and dry valleys.
Karren
Karren are small-scale and most typical landforms on karst surfaces formed mainly due to the chemical action of atmospheric water. Recent solution rate of carbonate rocks in the Dinaric area ranges from 30 to 100 m3 CaCO3/km2 (or the equivalent lowering of the surface in one million years for 30–100 m) as estimated by Gams (1966, 2000). Karren appear in various shapes and sizes, often on sloping surfaces. According to the morphogenetic principle, karren can be roughly divided into the following two groups: hydraulically controlled karren and fracture–controlled karren.
Hydraulically controlled karren occur on compact rocks. They are much shallower and the bottoms of their channels are smooth and usually properly elongated. It is considered that these small forms are exclusively linked to
limestones, i.e. they occur primarily due to the corrosive effects of atmospheric water. These karren most commonly occur on inclined surfaces when they occur in a linear arrangement (Fig. 1). Their depth in the Dinaric area may range from a few mm to about 0.5 m.
Fracture–controlled karren are formed by the expansion of systems of fractures and bedding planes through the chemical and partly mechanical action of water. In some cases, such karren continue into short elongated abysses, although these karren commonly descend down to 5 m. Carbonate rock surfaces dominated by karren are called karren fields.
Fig. 1 Hydraulically controlled karren on compact carbonate rock (key at the top of the photo may be used to assess the scale)
Littoral karren (micro pits) should be mentioned as a special group. They are represented by circular small forms. Unlike most other karren they are not produced by solution actions of atmospheric water. Their formation occur due solution actions of standing water. Littoral karren could be clearly observed in zones of seasonal lake–level oscillations where they may develop to high densities (Fig. 7b).
Karren can also appear beneath the soil cover in the epikarst zone. Epikarst represents the shallow subsurface zone that has a higher porosity than the deeper parts of the vadose zone. Soil thickness and quality, the content of biogenic acids and humidity concentration in the pedological layer have also influence on the development of epikarst, i.e. underground solution forms similar to karren on the terrain surface.
Sinkholes
Sinkhole (doline) is very frequent surface karst landform (Fig. 2). There are several types of this karst landform (solution, collapse, subsidence sinkholes), yet they all relate to differently shaped depressions in carbonate rocks, most often being funnel, saucer, bowl and well–shaped. Diameters and depths of these landforms also vary, most often in metric to hectometric scale. Their bottom is usually filled with soil, but can also be rocky or filled with coarse–grained material.
It is thought that the sinkholes are commonly associated with the system of fractures and faults. Sinkholes are often elongated along faults, and they also commonly occur at the intersection of two or more faults.
Sinkholes are most common in karst plateaus. Terrains densely pitted with sinkholes are often called polygonal karst (Williams 1972; Ford and Williams 2007). More words about sinkholes will be in the following sections.
Fig. 2 A funnel–shaped sinkhole in Kuči karst plateau
Uvalas
Uvalas are surface karst forms which are larger in size than sinkholes but smaller than poljes. These are depressions of hectometric size, often irregularly shaped. Bottoms of uvalas can be also covered with soil, most often with terra rossa. The longer axis of uvalas is often in parallel with the direction of faults and synclines. This is the case of most uvalas in the Dinarides which are mostly elongated in NW–SE direction.
Short and periodical surface water streams can emerge in uvalas; however, the drainage of uvalas usually takes place underground.
Poljes
Topographically closed karst depressions larger than sinkholes and uvalas are called poljes. The floors of poljes are usually flattened, but small hills may also occur. Polje is usually covered with lacustrine, moraine, glacio–fluvial and alluvial sediments and, apart them, thick complexes of other sediment rocks such as marl, clay and coal can be found. Poljes are most commonly predisposed by tectonics as suggested by the direction of the longer axis of most of the poljes of the External Dinarides (NW–SE).
The specific features of polje are reflected in the functioning of the hydrogeological system. The upstream of polje is characterized by strong karst springs from which surface water streams are formed. Polje drainage takes place through numerous swallow holes (ponors) usually located along downstream edges. In addition to water breaking out onto the surface, most poljes are also characterized by deeper permeable horizons through which
groundwater directly flows to the erosion basis. Poljes can often be flooded when water inflow exceeds the capacity of swallow holes. Flooding can also happen due to the rise of groundwater level above the level of polje bottom when outflows also appear in numerous estavelles.
Hydrotechnical reservoirs have been built in many poljes of the External Dinarides, preceded by extensive hydrogeological, engineering and geological research (Vlahović 1975; Milanović 2006). Due to high permeability of the limestones, great sinking losses of accumulated water represent the main problem even to this day. The opening of new swallow holes often occurs after charging and discharging of reservoirs.
Dry valleys
Dry valleys are represented by elongated karst depressions. The bottom of dry valleys is often intersected by crests of sinkholes and uvalas. Dry valleys lack surface water streams, except in rare cases when streams emerge only for a short period and for a short distance. In addition to dry (abandoned) valleys, karst terrains are also characterized by the valleys with permanent or temporary streams such as blind valleys, through valleys, gorges and canyons.
Subsurface karst forms
One of the basic features of karst terrains is their subsurface morphology encompassing different types of cavities developed in soluble rocks. In a large number of cases, sub–horizontal and sub–vertical conduits are intersected, thus creating caves systems. Caves commonly relate to subsurface karst forms that are large enough for human entry. However, subsurface karst forms should include all underground cavities, including the smallest ones which are created through solvent and mechanical action of water as well as through accompanying collapse actions. Therefore, subsurface karst forms also include all types of karst conduits and caverns of a different size and spatial orientation.
Caves develop mainly along faults, fractures, and bedding planes. These forms are especially linked to directions of fault intersections.
Based on hydrographic characteristics, Cvijić (1895a) distinguished two types of proper caves (referring to karst conduits in general): river caves and dry caves. Apart from hydrographic characteristics, these cave types also differ genetically. Cvijić considers that river caves were primarily formed by mechanical erosion of major streams, while dry caves developed due to the corrosive effects of atmospheric water. However, according to Cvijić (1895a) there are examples of dry caves created mostly by river erosion, and vice versa, some conduits of present caves with running water also genetically belong to dry caves. Cvijić further states that detailed research should be conducted in each case individually to determine if a cave genetically belongs to river caves or dry caves.
Although caves are typically influenced by chemical and mechanical water action, two main types of caves could be genetically distinguished by following Cvijić’s division and simplifying Bella’s classification (1998):
- corrosion (solution) caves–formed primarily by chemical erosion, and
- fluvial caves–formed mainly by erosion of strong turbulent streams.
Fluvial caves are characterized by various erosion forms such as potholes, subsurface canyons and natural bridges, as well as by the presence of fluvial materials (gravel and sand). In the case of corrosion conduits, larger erosion forms are absent, channels have more regular cross–sections, walls are usually coated with speleothems crust, and floor are often covered with the layers of soil, without larger rounded materials.
Corrosion caves are much more numerous and widespread, but generally they have far smaller diameters and length of individual segments as compared to fluvial caves. Solution conduits form a part of the subsurface karst network which is often well connected with systems of fractures. Such small conduits and caverns are often found while performing exploratory drilling when drilling equipment dropdown and loss of drilling fluid occurs. Solution forms also include primary vadose caves with blind vertical conduits (Fig. 3a) which are largely present mainly on karst plateaus. Bögli (1980) considers that primary vadose caves are caused by corrosive effects of rain and snow melt water. However, it is assumed that they can also appear due to the gradual lowering of groundwater level (drowdown vadose caves) (Waltham 1981). Vertical corrosion caves can be plain with a simple blind conduit or in step formation in which case their shape is partially adapted to major horizontal discontinuities. Vertical corrosion caves are found in all karst plateaus in Montenegro while the number of these caves in Trieste karst is extremely large (even 50–70 caves per 1 km2).
Fluvial caves are longer and have more spacious conduits compared to the conduits of solution forms. This
group includes caves with subsurface streams as well as swallow holes typically formed on the edges of uvalas and karst poljes, at the end of blind valleys or in sinking points of allogeneic streams that flow from impermeable terrains. Fluvial cave may be completely dry when they are above the maximum groundwater table, but many indicators suggest that flow existed in these caves. These caves were probably formed along the direction of existing solution conduits, which are additionally expanded by the action of strong turbulent flows with a bedload. The Slivlje swallow hole on the southeastern edge of the Nikšić polje (Montenegro) is a typical example of a fluvial cave. The opening of this swallow hole is about 25 m long and 18 m wide. A vertical shaft of a constant diameter descends 45 m from the surface. Via several steps divided by terraces with potholes, this cave descends 176 m (Fig. 3b). The Obod cave, from which the water of the Crnojevića spring (Montenegro) periodically flows out of, also belongs to this group of caves.
In the case of some caves it is difficult to discern whether their occurrence was caused due to chemical or mechanical erosion, since these forms were formed by the combined action of these two types of erosion. Erosion is also intensified by accompanying collapses of some parts of the ceilings and walls of the caves.
From the previously mentioned it can be concluded that karst conduits of different origins and size are interwoven through a karstic underground. Conduits formed by the soluble actions of water create a primary network spreading along fractures and faults, and are occasionally penetrated by vaster fluvial caves with a higher permeability.
It is very difficult to detect spatial distribution of caves and conduits, and hence a number of such phenomena remain unknown to researchers. New conduits with unknown surface entries and exits are often revealed when building tunnels or other structures. Speleological researches, if possible, still provide the most reliable data on the distribution and morphology of caves.
Fig. 3 a Cross–section of the solution cave Bujna Jama on the Piva plateau (after Lješević 2004), b cross–section of fluvial cave Slivlje on the southeastern edge of the Nikšić polje (after Petrović 1968)
The development of sinkholes according to the existing hypotheses
Since thorough research of karst geomorphology has begun, scientific understanding of the origin of sinkholes has been marked by inconsistencies and disagreements mainly related to the very creation of sinkholes and the timeframe. Generally speaking, disagreement has occurred usually between representatives of the idea of the erosion origin of sinkholes and advocates of the idea of the collapse of cave ceilings.
In his monograph "Karst", Cvijić (1895b) states that Austrian researchers (Tietze 1880) who investigated the typical karst of Kranjska Gora and the western part of the Balkan Peninsula, and linked geological knowledge of the Austrian and Montenegrin karst, concluded that sinkholes occur due to the collapse of cave ceilings. With these results they challenged earlier concepts present in Austrian literature at the time that sinkholes are formed by surface erosion. Contrary to this view is Mojisisovic’s opinion expressed in his research of western Bosnia (Mojsisovic et al. 1880). In his opinion, erosion landforms in pure limestone, which he calls karsttricker (probably referring to typical small sinkholes), belong to a group of geological organs. Diener (1886) generally agrees with this view and lists a number of evidence against the collapse theory. Although the research conducted in Kranjska Gora and in the Cevennes considerably contributed to the knowledge of karst terrains, researchers would later still come back with different conclusions. Kraus (1887) who conducted research in Kranjska Gora, concluded that collapse causes the creation of sinkholes and it occurs because groundwater dissolves and washes out limestone; also, all sinkholes are connected with caves. Martel (1890) came up with a completely opposite conclusion after conducting research of karst terrains in the Cevennes. According to Martel, small sinkholes are not connected with caves. From 40 examined sinkholes only seven were connected with well–developed cave conduits and a subsurface river, whereas only one sinkhole could have been caused by a collapse. At the time, the British and American researchers
disagreed with the theory of collapse, often stating that sinkholes are formed by dissolving action of atmospheric water and erosion along fractures and faults.
In his monograph "Karst", Cvijić (1895b) also presented his view of sinkhole formation, with the explanation that collapses do occur but are rare and most sinkholes are created due to the effects of atmospheric conditions. Temperature changes and chemical dissolution extended vertical and horizontal fractures in limestone; water is absorbed through these fractures which causes their additional expansion since water directly or indirectly dissolves limestone with its carbon dioxide. Due to different effects of surface water, these fissures expand and turn into a funnel. Cvijić called such phenomena normal doline (solution sinkholes) considering them the most common ones, but also acknowledging that this is not the only type of sinkhole formation.
Cramer (1941) contributed to distinction between solution (normal) and collapsed sinkholes. He compared topographic maps of various karst terrains around the world and made a morphometric description of the terrains concerned. Sinkholes and collapses, elongated along one direction, were often linked to groundwater streams. Stepanović (1965) states that a series of sinkholes along a dry valley axis indicates that a karst conduit goes along the same direction, whereas fresh collapses of sinkhole bottom show that a groundwater stream occasionally circulates through the karst conduit undermining the bottom of sinkholes from below.
Many authors accepted Cvijić’s view of the formation of solution sinkholes. Ninety years later, Paul Williams (1983) provided a model of the relation between sinkholes and epikarst which resembles Cvijić’s typical profile of sinkholes, as noticed by Ford (2007). Williams (1983) considers that terrain subsidence occurs gradually through a lasting process of diffuse recharge of epikarst hanging aquifers with atmospheric water which is accumulated in the subsurface zone and concentrically infiltrated deeper underground through a central vertical fracture (Fig. 4).
Also, it is often stated that solution sinkholes can occur following the erosion of overlay impermeable layer from which streams, while passing on karst terrains, gradually begin to sink and disorganize the terrains by forming a large number of sinkholes known as point–recharge
Fig. 4 Cvijić 1893 conception of the form of a dissolutional doline compared with Paul Williams’ 1983 model showing the relationship of doline and epikarst. Cvijić’ diagram is based upon a cross–section exposed in a railway cutting near Logatec, Slovenia. Ford (2007) believed that Cvijić would approve Williams of elaboration. Springer and the Environmental Geology, 51(5), 2007, 657–684, Ford D, Jovan Cvijić and the founding of karst geomorphology, Fig. 3, Copyright 2007. Reprinted with kind permission from Springer Science and Business Media
dolines (Sauro 2012). In recent years, certain "reconciling" theories have
emerged such as the theory of the expanded conduit (shaft) at the bottom of the epikarst zone (Klimchouk 2000), which is an attempt to reconcile the collapse and solution theories. According to this conceptual model, discharge of hanging epikarst aquifers results in the gradual expansion of vertical fractures from below and formation of a natural shaft closed with the upper layer of carbonate rocks. Due to the corrosion of terrain surface and shaft expansion from below, the upper layer gets thinner which at one point results in the collapse and opening of the shaft towards the terrain surface. According to this concept, the final phase includes shaping of the shaft sides and formation of a funnel–shaped sinkhole.
A number of scientists acknowledge several ways in which sinkholes can be formed and based on that they develop classifications which are in most cases similar to the one provided by Waltham et al. (2005). According to them sinkholes were classified as: solution, collapse, dropout, buried, caprock and suffusion sinkholes. However, these authors agree that solution sinkholes are dominant whereas natural collapse is an extremely rare phenomenon.
In recent years, sinkholes have been discovered on the deep ocean floor that has never been exposed to atmospheric conditions. Since each of the mentioned hypotheses assumes atmospheric water as an agent, through them is not possible to explain the genesis of these phenomena.
Dilemmas about the development of cave systems
Along with the emergence of ideas about the genesis of surface karst forms and carbonate aquifers, in the first half of the twentieth century, a number of hypotheses about the genesis of the caves appeared, from which Ford and Ewers (1978) highlight the following: vadose hypothesis, deep–phreatic hypothesis, and water–table cave hypothesis.
One of the first major hypotheses, known as the "vadose hypothesis", was advocated by the French speleologist Martel (1921). Having in mind the small solvent capacity of water enriched with CO2 from the atmosphere, Martel considered that such water could have an impact only in the vadose zone. He also believed that mechanical erosion caused by strong streams with bedloads has an important role in the expansion of channels only in the zone above water table. According to this hypothesis, the action of mechanical erosion does not affect the phreatic zone, which consists of only narrow channels created by the solvent capacity of slow flowing waters.
With the increasing number of surveyed caves, it was found that there were caves located deep below the regional water table. By using empirical evidence from cave maps and sections, the famous American geomorphologist Davis (1930) proposed a diametrically
opposite hypothesis (deep–phreatic hypothesis), in which the caves are developed slowly at random depths beneath the regional water table.
By recognizing the importance of soil CO2 as a booster of limestone solubility, there was a weakening of the previously mentioned two explanations. Swinnerton (1932) introduced a new hypothesis according to which the caves were formed along the water table, but also in the epiphreatic zone, which was similar to a previous explanation given by Cvijić (1918). Though, deep phreatic caves have remained an enigma for this hypothesis.
However, later experiments performed by Weyl (1958) found that infiltrating waters would quickly become saturated with calcite (after a few meters), so they do not continue to dissolve and thus caves could not be formed. It was a crisis period for each of the previously stated hypotheses. Since the existence of caves is a fact, an alternative explanation was necessary. In that period the effects of sulphides (Howard 1964) and mixing corrosion (Bögli 1964) on the development of caves were considered. But, based on newer laboratory experiments it was concluded that the dissolution rate of carbonates is non–linear and that there is large reduction in the rate as the chemical equilibrium is approached (Berner and Morse 1974; Plummer and Wigley 1976). The results have given rise to a revival of earlier hypotheses that are reconciled by the “for–state” model provided by Ford and Ewers (1978).
During the last two decades, a number of studies dealing with the genesis of caves have been performed (Knez 1996; Lowe 2000; Dreybrodt and Gabrovšek 2000; Palmer and Audra 2003; Worthington 2004).
Many dilemmas regarding the development of cave systems exist in today’s karstology. According to Ford and Williams (2007), no single theory of genesis has been able to encompass all caves except at a trivial level of explanation.
This section will review only two positions that arise from the above hypotheses:
- phreatic caves are developed slowly at random depths beneath the regional water table,
- dissolution and caves development cannot take place deep below sea level, i.e. below freshwater–saltwater interface.
One of the major scientific debates in this field is related to the clarification of the development of phreatic caves with conduits descending far beneath the water table. To this day, only a small number of this type of caves have been explored around the world, primarily because their mapping is extremely difficult since they are completely filled with water and speleo–diving is required for their research. Some of these caves have been investigated on the Montenegrin coast such as the Risan, Sopot, Gurdic and Ljuta caves (Dubljević 2001; Milanović 2007) with conduits descending below 100 m under the sea level. Potholes have been registered in the passages of the Risan cave (Milanović 2010) which can classify this type of cave in the group of fluvial caves. Thus, the discovery of potholes at the bottom of the phreatic caves has created
doubt in the previous explanations about the slow development of these channels.
Karst conduits detected in deep oil boreholes also pose a dilemma. For instance, exploration drilling in Romania registered caves filled with water at the depth of around 3,000 m. According to Ford and Williams (2007), some of these caves may be deep phreatic (bathyphreatic) caves but, as authors note, it is likely that many deep interceptions are of swallower types of caves that have been dropped downwards by tectonic activity. Also, deep drilling performed along the Adriatic coast (External Dinarides) showed that cavernous limestones exist in some areas at levels below 2,000 m under the sea level (Milovanović 1965). Thus for instance, due to the deep drilling performed near Ulcinj it was established that cavernous limestone–dolomite masses descend down to over 1,500 m under the sea level. At Bijela Gora, highly karstified (cavernous) limestones have been found at the depth from 1,560 to 1,579 m. In a deep borehole in Ravna Korita, periodic and often intensive loss of drilling fluid occurred up to the depth of 2,000 m. In Rovinj surroundings, cavernous dolomites lie in the range of 1,350–2,330 m where a loss of drilling fluid has been registered. By performing deep drilling on the island of Vis, karstified layers of limestone, dolomite and limestone breccia, in depth range of 732 to 2,040 m beneath sea level, have been registered (periodically losses of drilling fluid would occur). Despite extensive research, no indicators of hypogene speleogenesis caused by water containing H2S have been detected in this area. What is puzzling about this region is that geologists (Waisse 1948; Grubić 1975) believe that in the past it was affected by orogenesis, i.e. terrain uplifting instead of lowering. So the question arises as how the karstification of subsurface zones lowered to such great depths.
New concept
Numerous shortcomings of the existing explanations of karst development have created a need to establish a new conceptual model that would explain the karst phenomena in a more acceptable manner.
This section will first explain the terms related to submarine dissolution (lysocline and calcite compensation depth), followed by the elaboration of the arguments in favour of the hypothesis that the majority of karst landforms have been formed underwater, and an explanation of the mechanism of their formation.
Lysocline and calcite compensation depth
Shallow seawater is generally supersaturated in calcite but as depth, i.e. hydrostatic pressure increases calcite saturation of seawater decreases. Thus, calcium–carbonate shells are not subject to dissolution in seawater until they sink below the lysocline, which represents the depth in the ocean below which water becomes aggressive to calcite
and the rate of calcite dissolution increases dramatically (Berger 1967; Wise 2003).
Below the lysocline there exists the calcite compensation depth (CCD) which is the ocean depth at which the rate of supply of mineral calcite (CaCO3) equals the rate of its dissolution (Bramlette 1961; Pytkowicz 1970; Wise 2003). In the present conditions, below such depth deposition of carbonate sediments is not possible. Therefore, the lysocline depth and calcite compensation depth depend on the solubility of calcite or, more precisely, of the parameters that influence its dissolution such as temperature, hydrostatic pressure and chemical composition of water, especially the content of carbonate ions and dissolved carbon dioxide (CO2) in water. Solubility of calcium carbonate increases with decreasing temperature and increasing hydrostatic pressure. It also increases with decreasing content of carbonate ions and increasing partial pressure of CO2.
The depth of the lysocline varies and usually ranges between 3,700–4,000 m (Wise 2003). The variations of CCD have been also observed, but it can be stated that its average value is around 4,500 m. Variations of these depths are the function of the circulation patterns within the oceans (Berger 1967).
The genesis of sinkholes
This subsection first elaborates on three facts which indicate the underwater formation of sinkholes followed by the explanation of the genesis of these phenomena according to the new concept. In this paper, the term sinkhole is mainly related to solution sinkhole i.e. doline.
Existence of sinkholes deep below sea level
The development of new techniques intended for detailed bathymetric surveying of topography and features of sea floor (multibeam echosounders) led to the discovery of a number of closed circular depressions that resemble karst landforms such as sinkholes, uvalas and poljes. These phenomena are also often registered at great depths so it is considered that they could not have been formed on the surface of terrains due to the influence of atmospheric conditions (Land et al. 1995). A rather spacious underwater area clustered with sinkholes and uvalas of multi–kilometre diameters (Fig. 5) formed in carbonate sediments has been detected near the Galapagos Islands (0°45’–0°55’S; 89°50’–90°50’W), at the depth between 1,500 and 2,600 m (Michauda et al. 2005). The dispersion pattern of the underwater sinkholes detected in this area is very similar to the dispersion pattern of surface sinkholes, as indicated by nearest neighbour indexes (NNI) (Clark and Evans 1954). NNI calculated for the mentioned area clustered with underwater sinkholes amount 1.36. Nearest neighbour statistics have been done for many terrains worldwide where the polygonal karst is developed on the surface, as clearly presented by Ford and Williams (2007). Based on this data it can be observed that NNI for 10 out
Fig. 5 Close–up of the southern flank (a) and on the northern flank (b) of the Central Carnegie Ridge near Galapagos Island showing a densely packed field of circular depressions (grid size 200 m; processed from Caraibes software, IFREMER). Bathymetric interval is 10 m. Reprinted from Marine Geology, 216/4, Michauda F, Chaberta A, Collota J, Sallarèsa V, Fluehb E, Charvisa P, Graindorgea D, Gustcherc M, Bialasb J, Fields of multi–kilometer scale sub–circular depressions in the Carnegie Ridge sedimentary blanket: Effect of underwater carbonate dissolution? 205–219, Copyright (2005), with permission from Elsevier
of 14 processed areas has values in the range between 1.0 and 1.4. Based on this data, it can be clearly concluded that the dispersion pattern of the sinkholes formed by underwater dissolution is almost identical to commonly detected dispersion patterns of surface sinkholes. This may indicate that the processes by which these forms were created were identical.
Occurrence of sinkholes on the rocks with relatively homogeneous hydraulic conductivity
The explanation of the genesis of sinkholes due to corrosive action of atmospheric water requires the existence of an enlarged fracture in the sinkhole centre along which water concentrates and infiltrates into deeper
zones. Since sinkhole bottom is covered with soil layer, this fracture is almost impossible to register from the surface. Morphology, anisotropy and karstification degree of carbonate rocks can be properly observed at locations where sinkhole is intersected by excavation or road cut. Excavations in the floors of many solution sinkholes have revealed nothing more than networks of narrow fissures (Waltham et al. 2005). At many road cuts it can be clearly observed that it is the case of a relatively isotropic environment with regards to fissuring of rock mass (Fig. 6), i.e. the central conductive zone does not exist which is, by many researches, a necessary condition for the genesis of sinkholes (Cvijić 1893; Williams 1983; Klimchouk 2000). Therefore, enlarged vertical fractures or swallow holes (ponors) can be formed at the bottom or edges of
Fig. 6 The cross–section of the solution sinkhole exposed in a road cutting on the Herceg Novi–Trebinje road. The polygonal karst is developed in this area
sinkholes because terrain morphology has enabled water collection and accumulation. However, the emergence of expanded fracture is not a necessary condition for the genesis of dissolution sinkholes; expanded fractures and swallow holes are actually often a secondary phenomenon caused by the drainage of sinkholes. On the other hand, enlarged fractures and vertical vadose caves appear at many places in karst terrains which are not linked to depressions. Entrances of vertical vadose caves can be located even on the crest between two sinkholes, which is the case with Bujna cave (25 m deep, Fig. 3a) in the Piva region (Lješević 2004). Therefore, in this case sinkholes are not concentrated around the main conductive zone (vertical vadose cave) but are formed on more compact rocks. The origin of elongated sinkholes along faults falls under a separate case which is easier to explain with the new concept of underwater corrosion compared to previous views on the genesis of sinkholes caused by the action of atmospheric water. This will be, however, discussed more in the section dealing with the genesis of uvalas and poljes.
Existence of microforms of sinkholes
Littoral karren (micro pits), formed due to the corrosive action of lake water, occur in many coastal zones. They are usually densely clustered in a polygonal pattern and morphologically reminiscent of polygonal karst consisting of sinkholes at many karst plateaus (Fig. 7). Vajoczki and Ford (2000) studied the appearance of littoral karren in the coastal part of the Georgian Bay (Lake Huron, Canada) to the depth of 25 m and they found an exceptionally good correlation between the depth of littoral karren and water depth (r2 = 0.93). Interestingly enough, the researchers of the previously mentioned underwater sinkholes near the Galapagos Islands (Michauda et al. 2005) also managed to establish some correlation between ocean depth and the depth of karst depressions. Based on this, it can be concluded that the processes governing the formation of littoral karren and underwater sinkholes are very similar, although quantitatively different. Littoral karren are distributed on compact rocks more or less densely and
mainly in polygonal pattern, whereas rocks that are cut by a fissure, karren are distributed along that predisposed direction (Simms 2002). Such is the case with the distribution of sinkholes and uvalas in karst terrains.
Discussion about the development of sinkholes
Based on these facts, it can be concluded that the formation of circular karst depressions most probably occurred underwater below the lysocline, where the dissolution of carbonate sediments is increased. This hypothesis of underwater dissolution of carbonate sediments may have been first put forward and favoured among several others (pockmark origin, sediment creeping, paleo–topography, effects of subbottom currents, and both marine and subaerial karstic origins) by the researchers who studied karst depressions on the ocean floor near the Galapagos Islands (Michauda et al. 2005). However, their explanations focused only on the genesis of the submarine sinkholes that are now beneath the ocean. According to the new concept, the development of the karst depressions which are now on the surface occurred in the same way, during the period when the surface was still under the sea.
Most probably the processes of deposing and dissolving the rocks altered successively. In this way it is also possible to explain the development of fossil sinkholes and other paleokarst forms. The lysocline was lowering due to the regression of the sea; hence the deepest karst depressions are reasonably found under the present sea level, i.e. in areas that have been most exposed to underwater dissolution.
According to the new concept, the impact of atmospheric water on the development of karst landforms is limited only to the development of hydraulically controlled and fractured–controlled karren (not including the littoral karren). Actually, atmospheric water has a dominant influence on the expansion of existing fractures only in the surface zone, since atmospheric water has the highest potential for dissolution at the beginning of infiltration. The result of this, is the existence of a lower border of epikarst to which the effect of atmospheric water is evident. This border is usually located to the depth of a few meters. It is often clearly distinguishable from the deeper parts of the carbonate rocks which have a much lower degree of karstification. The appearance of epikarst (subcutaneous zone) was well observed by Williams (1983), but according to the new concept, epikarst should not be linked to the genesis of sinkholes since sinkholes can also occur in places where epikarst is not developed. Epikarst represents a zone consisting only of karren (mostly fractured–controlled karren) which can often be covered with soil.
Collapse sinkholes are not common, and though the final rock collapse may be almost instantaneous, natural collapse events are extremely rare (Waltham et al. 2005). Karst collapses in solid rock masses are the result of a collapse of previously formed cave passages with ceiling extending almost to the surface. Suffosion and dropout
sinkholes are formed through the processes occurring in the overlay sediments deposited over carbonate rocks with well–developed karstic network. Hence, the genesis of these landforms should be linked to the secondary processes taking place after a more dominant phase of karstification.
The genesis of uvalas, poljes and dry valleys
As mentioned earlier in this paper, uvalas and poljes are typically closed karst depressions larger than dissolution sinkholes. By studying poljes in the western Bosnia and Herzegovina, Cvijić (1900, 1918) realized that all major poljes are linked to the tectonic rifts, faults or the syncline axis mostly of the Dinaric direction (NW–SE). He also realized that tectonic depressions are just a starting point for erosion action because all tectonic forms are considerably narrower than the poljes which are formed by the marging of uvalas. Therefore, the new concept acknowledges that uvalas and poljes are commonly (but not always) linked to faults or structural tectonic forms. However, according to the new concept, the corrosion which expanded and shaped tectonic forms is being linked to a far more intensive underwater dissolving action as described earlier in this paper. Clay sediments with marl and coal interbeds often cover the bottom of poljes in the Dinarides, which is another indicator that these karst depressions used to be underwater.
Dry valleys are nothing else than a series of linearly arranged and partly shaped sinkholes and elongated uvalas which morphologically resemble normal river valleys. The major difference is that dry valleys do not have a continuous slope; instead their bottom is intersected by crests of uvalas and sinkholes. Such series of elongated uvalas and sinkholes are classified as a blind valley if it does not descend to the erosion basis (most common case) but ends with divide of the lowest uvala in the series whose edges are covered with swallow holes. The areas around the swallow holes distributed across the downstream edge of blind valleys, uvalas and poljes are often accompanied by potholes (Cvijić 1960). This indicates the presence of strong and turbulent water movements conditioned by a large hydrostatic pressure of water that karst depressions
were filled with. These flows could also have a certain influence on shaping of karst valleys.
The genesis of subsurface karst forms
Karst conduits were registered while performing exploration drilling on the Adriatic coast at depths of more than 2000 m below sea level (Milovanović 1965), as described above. Therefore, the genesis of these corrosion karst conduits also cannot be explained by dissolvent action of atmospheric water or water containing H2S. In the conditions of underwater dissolution below the lysocline, the expansion of the fractures earlier formed by tectonics is also possible; hence the primary network of karst corrosion conduits was most likely developed underwater. In the following phase, while the water was regressing, water retention in karst depressions occurred, and once the necessary hydraulic gradient was reached, water began to run off through earlier formed corrosion conduits. Due to the turbulent movements of groundwater, some corrosion conduits were extended and fluvial caves appeared along their direction, usually much spacer than the conduits of the primary network.
Water reduction from karst depressions first causes the occurrence of swallow holes in the upper part of edge of uvala or polje, extends the upper channels of the subsurface karst network and runs off downstream at the level of lower water that has withdrawn faster (Fig. 9d). Further withdrawal of flood water leads to the formation of lower swallow holes, extension of the lower horizon of conduits, emergence of springs and possibly spring caves at lower levels as compared to the previous discharge point. The lowest swallow holes along the downstream edges of uvalas or poljes are often functional to this day (Fig. 9e). The lowest discharge zone probably occurred once the hydraulic gradient was formed and it was further extended once the water had completely withdrawn. The lowest fluvial caves that are most recent are typically the largest in size, and they can descend even lower than the level of the discharge point while forming phreatic caves with loops and vauclusian (ascending) type of springs. In the case of conduits of descending type, cave exits are typically characterized by potholes, the collapse of ceilings and the occurrence of similar forms indicating that water
Fig. 7 a Polygonal karst with dense distribution of sinkholes on the mountain Golija (Montenegro) photographed from an aircraft; b Littoral karren (micro pits) on the shore of Skadar Lake (Montenegro)
circulated through these channels under extremely high pressures.
In areas with a series of uvalas and poljes distributed in step formation, swallow holes typically occur on the downstream edges of almost all major karst depressions. The appearance of swallow holes is not necessarily linked to their successive (upstream or downstream) shift along dry valleys as it has been explained by many authors, starting with Cvijić (1960). Their formation is more realistically linked to the previously described process which first occurred at the edges of uvalas at higher altitudes, then at the edges of lower uvalas or poljes, ending with the lowest depression located just above the regional erosion basis.
Vauclusian (ascending) type of springs and phreatic caves with loops are formed in areas with a high hydraulic gradient which enables water from upstream karst depressions to flow through corrosion conduits to below the sea level, i.e. below the lower water level. At certain depths, when the hydrostatic pressure of lower water was high enough to prevent further penetration of phreatic caves, groundwater is forced to break out onto the surface and create lifting forms of exit passages.
A connected system of reservoirs "Višegrad" and "Bajina Bašta" on the Drina River could be considered as an experimental evidence of a rapid expansion of karst conduits due to changes in hydrostatic pressure (Fig. 8). These reservoirs are arranged in step formation. The "Višegrad" reservoir is located upstream of the "Bajina Bašta" reservoir. The "Višegrad" dam is 79.5 m high and the difference between the levels (∆H) of these two reservoirs is theoretically within the range from 28.8 to 69 m (Vučković et al. 2004). The gorge of the river Drina in the area of the "Višegrad" dam cuts into limestones and dolomites considered to be reasonably impermeable prior to the dam construction. Since the Drina riverbed represents the regional erosion basis below which the degree of karstification should be small, greater losses from the reservoir were not expected. However, after the initial charging of the reservoir three submarine springs emerged downstream of the dam. Infiltration losses were estimated to be 1.4 m3/s after a year, around 6 m3/s after 7 years, and around 15 m3/s after 20 years following the construction of the reservoir "Višegrad" (Milanović 2009). Hydrogeological and speleodiving research (Milanović 2009) discovered a swallow hole with 4.5 m diameter at the bottom of the "Višegrad" reservoir near the dam, which was not registered in earlier research performed prior to the construction of the dam. The discharge through a number of submarine springs occurs below the level of downstream reservoir, about 300 m away from the dam. Mathematical modelling estimated that karst conduits have knee shape and they descend below the dam body and the grout curtain to the depth of around 150 m. Some of these conduits were also confirmed by drilling. The author of the Report concludes that the formation of swallow holes and subsurface conduits occurred due to the expansion of faults intersecting this area (Milanović 2009).
This example clearly indicates that the extension of the previously formed primary network of karst conduits and formation of phreatic caves are possible even if the hydraulic gradient is much lower than the one which might have existed between the upstream karst depressions and downstream vauclusian springs.
Fig. 8 Schematic overview of the genesis of the extend karst conduit beneath "Višegrad" dam. a Karstified porous environment consisting of the primary network of karst conduits prior to the construction of the "Višegrad" dam on the Drina River; b swallow hole, fluvial cave and sublacustrine spring formed in post–constructional period (1985–2009) as a result of increased hydrostatic pressure in the back of the dam
The presented concept of the speleogenesis could also be applied to simplify the explanation of the existence of overburden karst, i.e. commonly called paleokarst. In addition to karst depressions, overburden karst can also consist of the primary network of corrosion karst conduits which is often detected by performing deep drilling. Underwater dissolution of carbonate sediments has been often "competing" with the sedimentation of carbonates or other sediments. Hence, some periods were dominated by dissolution when karst depressions and networks of corrosion conduits were formed, followed by periods of sedimentation when previously formed karst forms became covered. In this way it is also much easier to explain the underwater formation of carbonate bauxites.
Dissolution of carbonate sediments below the lysocline is the chemical process completely different from the process so-called hypogene speleogenesis caused by water containing H2S. The link between these two types of karstification has not yet been established, but in any case it is necessary to conduct more extensive research in order to further clarify this issue.
Karst development sequences according to the new concept
The development of the karst as known nowadays was not influenced by atmospheric conditions; its development mainly occurred beneath the water surface having been caused by the subsequent actions of strong turbulent flows resulting from water withdrawal.
Before the start of the formation of karstic networks by underwater dissolution of carbonate sediments, it was necessary that the fractured carbonate rocks occur at great depths below sea level, i.e. below the lysocline. The authors of the previous hypothesis of the underwater dissolution of carbonate rocks below the lysocline (Michauda et al. 2005) also indicate that, prior to the intensive karstification process, a heterogeneous porosity network in the carbonate sediments already existed.
The following might be an explanation of one of the possible ways as to how consolidation and appearance of fractures within carbonate sediments occur before the beginning of underwater dissolution. Carbonate sediments deposited above the lysocline were initially unconsolidated, with a porosity of about 40–80 %. Immediately after deposition, submarine cementation began; moreover, the deposition of new layers of carbonate sediments caused the compaction of lower layers, so that these sediments became more consolidated. By tectonic movements, the fractures and faults were formed within the lower consolidated layers. Due to the quick and significant increase in water level, carbonate sediments became exposed to a very intensive underwater dissolution below the lysocline. The unconsolidated upper layers were the first to be rapidly dissolved. From the moment of exposure of the lower consolidated fractured carbonate sediments to underwater dissolution, the primary network of karstic channels and elongated karst depressions start to be created as mentioned in previous sections. Dissolution will continue to occur until some other insoluble materials cover carbonate sediments or until the sea level decreases to the level where carbonate sediments are above the lysocline again. Cementation and compaction of carbonate sediments could partially continue above the lysocline, and also partially after exposure of carbonates to atmospheric conditions. This could result in closure of one smaller number of the karstic conduits.
Underwater dissolution and the genesis of karst, which is the main topic of this paper, most likely occurred in a few sequences, as shown and described in Fig. 9.
Conclusion
Compared to the existing hypotheses, the presented concept provides a more simple explanation of the genesis of karst landforms and karst in general.
Most karst landforms developed underwater due to the dissolution of carbonate sediments at greater depths. Solvent action of atmospheric water causes only the formation of the majority of smaller karst forms such as
some karren. By determining the deepening rate of karren and their average depth in compact rocks, the exposure time of carbonate rocks to atmospheric conditions could be assessed.
Dissolution sinkholes exist only in the areas that have been exposed to underwater dissolution. Therefore, their occurrence is not realistically expected in places which are the result of the later erosion (such as steep canyon sides),
Fig. 9 Karst development phases according to the new concept a tectonically disrupted carbonate rocks are located at greater depths beneath the ocean, more precisely at depth below the lysocline; b creation of sinkholes and uvalas by underwaterdissolution. Extension of fractures and inception of the primary karst network of corrosion conduits; c continuation of dissolution. Uvalas are being extended and poljes are created. Further extension of fractures occurs; d water withdrawal. Highhydraulic gradient is created by water retention in karstdepressions. Swallow holes are formed as well as fluvial caves inthe present aeration zone, and upper springs occur downstream; ecomplete water withdrawal. The lowest horizon of fluvial phreatic caves has been previously formed. Deposited sediments remain in poljes. Water of atmospheric origin is infiltrated through already formed conduits and creates karst aquifers as known today
which could not have existed beneath the lysocline. This hypothesis could be checked by mathematical modelling of the genesis of karst landforms by applying the new concept and comparing the modelled results with the actual situation. Many authors (Eraso 1986; Huntoon 1995; Bakalowicz 2005; Supper et al. 2008) think that the current mathematical models have given unsatisfactory results.
So far this concept has touched only on some of the most important fields of karstology and opened the space for different views on this subject. At this point, the presented concept is just a hypothesis that needs to withstand the test of time. This subject matter might be better elaborated on by including more researchers that would utilize numerous facts from their research fields.
Unlike many other scientific disciplines, karstology cannot boast a great success in the twentieth century. It is believed that the establishment of new conceptual model would speed up the development of this subject area and improve the principles and methods for solving a number of practical issues such as: modelling of the development of subsurface karst network and groundwater circulation, simplifying detection of aquifers, as well as deposits of bauxite, oil and gas within carbonate rocks.
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